The electrical power supply for electrical equipment onboard an aircraft is traditionally provided by an electric current power supply cable and by a current return network constituted by the aircraft structure. The current return network may be considered like the general ground plane of the aircraft.
In an aircraft in which the fuselage elements and the primary structure (panels, frames, stringers) are made from a metallic material, the current return is made by the presence of many interconnected structural parts. The large number of interconnections from one end of the aircraft to the other makes it easy to transport current.
The design of recent aircraft makes use of composite materials for some parts of the external or internal structure of the aircraft in order to reduce aircraft weight.
The composite materials used such as carbon or resin and fibre alloys have low electrical conductivity and thus cannot be used to form a current return network. In this case, the current return is made using a network of high conductivity metallic structures called the ESN (Electrical Structural Network) network, located in the pressurised zone of the fuselage and composed largely of metallic structural elements.
FIG. 1 shows a global view of an ESN network 2 of part of an aircraft 1. FIG. 1 more particularly shows that the ESN network 2 of an aircraft is a meshed and redundant network because each element in the ESN network is connected to at least two other elements in the network. Redundancy of the ESN network is necessary to respect safety standards set down by civil aviation authorities.
More precisely, the ESN network is composed of structural metallic elements and metallic elements of junctions that guarantee electrical continuity between the different structural elements. FIG. 2 shows a straight section of the fuselage of an aircraft 1 comprising elements made of composite materials 3 and an ESN network.
There are two types of structural elements in an ESN network:
primary structure elements 4a of the aircraft, for example metallic frames, metallic cross-pieces, metallic seat rails, aircraft floor(s);
secondary structure elements 4b of the aircraft, that include metallic cross piece elements to support electrical equipment in the aircraft, for example such as computers.
There are also two types of junctions:
flexible electrical junctions of the “metallic braids” type, used for example to make some connections between primary and secondary elements in the ESN network.
electromechanical junctions that perform both mechanical and electrical functions. These are junctions based on rivets or screws and bolts, between two structural elements in the ESN network. In some cases, an electromechanical junction may be reinforced by a flexible junction.
An ESN network also comprises standard parts 4c for example such as U or I metallic components acting as supports for electrical cables of the aircraft. Such supports are called “raceways” in aeronautical language. Standard parts are connected to the primary or secondary structure elements of the network through flexible electrical or electromechanical junctions as defined above.
As shown in FIG. 2, the various structural elements in the ESN network are electrically connected at the composite/metal interfaces 5, to elements 3 made of composite materials that form part of the structure of the aircraft.
Electrical continuity between the different metallic elements making up the ESN network is only obtained after a so-called metallisation operation done during the aircraft production cycle.
Corrosion protective paint usually covers all metallic elements in the ESN network. Obviously, this paint is not conducting. Furthermore, intermediate mastic is often present at electromechanical junctions between the different structural elements in the ESN network to prevent contact wear phenomena.
The metallisation operation consists of stripping the paint and if necessary removing the intermediate mastic from a contact surface to locally expose the metallic element 4a, 4b being considered.
Small residual quantities of mastic or paint at a junction between two elements in the ESN network are sufficient to degrade the quality of the electrical contact and therefore the efficiency of the ESN network.
It can then be understood that junctions between the different elements in the ESN network have to be tested. This test must be possible when assembling the ESN network, or when repairing it or during aircraft inspections.
It is considered that an electrical junction in the ESN network has a satisfactory conduction performance if it is capable of transporting the current intensity for which it is specified for a sufficiently long time, in other words at least 60 seconds, without major overheating or degradation of its impedance. Electromechanical or flexible junctions in an ESN type network must be capable of transporting at least 50 A rms at a frequency of the order of 400 Hz.
Impedance measurement methods used traditionally in electronics, for example using a milli-ohmmeter or a current generator associated with a voltmeter, are not appropriate for testing junctions between two elements in an ESN network because the network is meshed and redundant.
FIG. 3 shows the implementation of the so-called 4-wire method, also called the Kelvin method, to measure the impedance of an electromechanical junction AB in the ESN network using a current generator 6 associated with a voltmeter 7. For obvious reasons of conciseness, the ESN network is shown diagrammatically in FIG. 3 and only electromechanical connections between the primary structure elements A, B, C, D, E of the network are illustrated. Resistances R1 and R2 are resistances due to interfaces between the ESN network and composite elements of the aircraft.
In applying this method, the current injected by the current generator 6, greater than or equal to 50 A, is distributed in the different branches formed by the different elements A, B, C, D, E and the composite elements of the aircraft, prorata to the resistances of each branch.
Consequently, the 4-wire method cannot test the performance of the junction AB because its application provides the measurement of the impedance of the junction AB in parallel with the rest of the network and not the junction AB alone.
Furthermore, this method is incapable of controlling the current intensity injected into the ESN network and its use can damage the composite structure that is particularly sensitive to passage of a current, particularly when the composite material used is carbon.
The invention discloses a method of testing the performance of an electrical junction between two metallic elements in an ESN network, which does not deteriorate the composite structure of the aircraft when used.